Magnetic resonance imaging has a number of capabilities and attributes that distinguish it from other diagnostic modalities. In addition to its potential ability to monitor cell metabolism on a molecular level, an important feature of the MR signal is its sensitivity to bulk motion such as blood flow in living tissue. Depending on the specific pulse scheme used and on the nature of the pulses administered, the image of flowing fluid may be either enhanced or reduced in intensity relative to a stationary ambient environment. The flow dependence of the MR signal is one of MR's unique properties whose exploitation for the study of the cardiovascular system has only recently begun.
Many approaches have been proposed for measuring or imaging flow. One group of MR methods for imaging flow are commonly referred to as time-of-flight methods. In one well-known time-of-flight method, a selective saturation recovery spin echo pulse sequence (hereinafter referred to as "SSRSE") is utilized to provide images which are enhanced in the area of flow. Although this method has proven highly successful in imaging venous flow, this method and others fail to satisfactorily distinguish arterial flow from its surrounding stationary tissue. Apparently, the pulsatile nature and high velocity of flow in arteries (resulting from flow surges caused by cardiac contractions) severely impairs refocusing the spins associated with the microscopic magnetic moments caused by the slice-selection magnetic gradients applied during MR imaging. The impairment of refocusing by flow effects leads to irreversible dephasing of the spins. Because refocusing of the spins is severely impaired by fast flow, the image from arterial flow is not necessarily of enhanced intensity as is slower, venous flow and, therefore, arterial flow may not be distinguishable from surrounding stationary tissue. A further complication is the circumstance that arterial flow is pulsatile, i.e., the flow velocity varies over the cardiac cycle.
Because of the lack of known methods to specifically image arterial flow, the MR image has been unable to give a reliable indication of the nature of a particular vessel. Since arterial stenosis from atherosclerotic buildup is a common pathological condition and since cardiovascular disease is the major cause of death in this country, an accurate image of arterial flow is important. Moreover, arterial stenosis commonly does not become symptomatic until it is life threatening. Because early detection of arterial stenosis may allow treatment of the blockage with drugs or other means less radical than surgery, early detection of abnormalities in arterial flow is diagnostically important.